One technology that is used for display systems is based on a digital micromirror device or DMD. Such systems are commercially available from Texas Instruments, Inc. under the tradename DLP (Digital Light Processing). Today's DMD-based projectors typically use a single DMD with the illumination light applied in a color sequential manner (called Field-Sequential-Color or FSC). A rotating color wheel is typically used. Light is applied as red (R) then green (G) then blue (B) with each color sequentially applied so that the color completely fills the DMD.
With this system there are time intervals during each frame where the colors are briefly a mix of some pair of the RGB primary colors. This occurs when the spokes of the color wheel are passing through the lamp output light cone. This spoke light is used in today's projector via Spoke-Light-Recapture (SLR), as described in U.S. Pat. No. 6,324,006 which is incorporated herein by reference. So, for an all white screen, today's FSC projectors have the DMD on throughout the R, G, and B segment times as well as during all of the spoke times.
However, when a single color is applied to the DMD in a FSC system, the complement colors are being reflected by the color wheel and this light is scattered and lost. The color wheel is only transmissive for the color currently being applied to the DMD. For an all white screen approximately two thirds of the screen lumens are lost since during each color only ⅓ of the visible spectrum of lamp light is transmitted through the color wheel to the DMD. For example, when red light is being passed both green and blue are reflected.
One way to quantify the light loss at the color wheel is to define the term “Color Wheel Efficiency” or CWE. CWE tells how much light is lost due to the color wheel in the DMD system. CWE is the ratio of screen lumens with the color wheel installed in a projector to the screen lumens with the color wheel removed from that projector. If the color wheel were 100% transmissive you would have CWE of 100%. But for a typical FSC RGB color wheel CWE is only 33%. So for a typical FSC projector with a RGB color wheel 67% of the light is lost. This is a very significant light loss and a method is needed to recapture this lost lamp light. Scrolling Color (SC) optical concepts are devised to try and recover some of this discarded light.
Throughout this discussion the general scrolling color DMD system will be referred to as a SC projector. One SC method of recapturing the lost lamp light, which is described in co-pending application Ser. No. 09/705,467, filed Nov. 3, 2000, is called Sequential-Color-Recapture or SCR. This concept will be referred to in this disclosure as SCR, which is one type of optics for a SC projector. However, an SC projector can use other scrolling color concepts such as a rotating prism.
In today's FSC projectors a white (clear) segment is also used in addition to transmissive R, G, and B segments. This additional segment gives a lumens boost for white and grays in images since the white segment passes the full spectrum of lamp light.
For today's FSC projectors, the DMD is illuminated with mixed-color light approximately 20% of the time. This “spoke light” cannot be used to create pure R, G, or B light on the screen. With SLR, this spoke light is mixed with the W segment white light so that an effective W segment is created that is larger than the physical W segment. So for a RGBW color wheel, like with a RGB color wheel, no light is lost due to the spokes. For an all white screen with today's FSC projectors, which include a W segment, the DMD is on throughout the R, G, B, and W segment times as well as during all of the spoke times.
The addition of a W segment typically will boost screen lumens about 35% over a wheel that contains only R, G, and B segments. This 35% boost assumes a W segment size of approximately 52°, out of a 360° wheel, which is common in today's FSC projectors. This results in an increase of CWE from 33% for an RGB wheel to 44.5% for an RGBW wheel (33%×1.35). It is desirable, however, that the CWE be raised even further.